Many currently available prosthetic and orthotic ankle-foot mechanisms do not allow ankle motion. Rigid ankle prosthetic and orthotic ankle-foot devices generally attempt to replace the actions of the biologic ankle-foot system through deformations of their materials and/or by utilizing rocker shapes on the plantar surfaces. The prosthetic and orthotic ankle-foot devices that do incorporate ankle motion usually allow rotational motion about a single angle that does not change without mechanical adjustments of the prosthesis or orthosis. Some of these devices use springs and/or bumpers to store and release energy and return the device's ankle joint to one “equilibrium” point. This single and constant “equilibrium” point can result in good function on level terrain and when using shoes of one particular heel height (heel and forefoot sole differential). However, problems can arise when walking on different terrain or when using shoes of different heel height. The heel height problem can be fixed using a change in the alignment of the prosthesis. However, this is not a simple task and one that does not happen automatically.
A patent issued to Wayne Koniuk (U.S. Pat. No. 6,443,993 B1, “Self-Adjusting Prosthetic Ankle Apparatus”, issued Sep. 3, 2002) discloses a device that will adapt to various terrains and to shoes of different heel height. However, Koniuk's design does not appear to have energy storage and release properties, utilizes more sensing devices than the proposed design, and does not appear to give plantarflexion at late stance. Koniuk's design is based on damping control of the ankle joint whereas the proposed device is based on the control of stiffness about the ankle. Damping removes energy from a system whereas stiffness can store and release energy to a system throughout a loading and unloading cycle (that is, a walking cycle).
Recent research has suggested that roll-over shape, the effective rocker shape that the ankle-foot system conforms to between heel contact and opposite heel contact, is an important characteristic for walking. Hansen ((2002); “Roll-over Characteristics of Human Walking With Applications for Artificial Limbs.” Ph.D. dissertation, Northwestern University, Evanston, Ill.) found that the able-bodied ankle-foot system adapts to several walking conditions to maintain a similar roll-over shape and that its roll-over shape changes predictably when walking on inclined or declined surfaces. Specifically, able-bodied ankle-foot systems are capable of automatically adapting to differences in shoe heel height and to different surface inclinations. Current prosthetic ankle-foot mechanisms cannot automatically adapt to these conditions. Recently, prosthetic devices have come onto the market that claim adaptability to sloped surfaces, yet these devices have their limitations. For example, the Echelon (Endolite North America, Miamisburg, Ohio, USA), which has a series combination of spring and damper, is claimed to be able to self-align for varied terrain (as stated in product literature). This combination permits the foot plate to rotate through a nine degree arc (six degrees in plantarflexion and three degrees in dorsiflexion) before reaching the physical limits of the viscoelastic range of motion, at which point it transitions to a predominantly elastic range of motion produced by deflection of the foot plate.
On inclined and declined surfaces, the effective ankle angle at which the viscoelastic range transitions to the elastic range remains unchanged. Thus the Echelon does not mimic the change in equilibrium point of the anatomical ankle. Instead, it tolerates or accommodates changes in surface orientation.
The Motionfoot (Motion Control Inc., Salt Lake City, Utah, USA) also utilizes a series combination of spring and damper. While the Motionfoot has a greater range of viscoelastic motion than the Echelon, it also provides a single equilibrium point on sloped surfaces (that is, a single ankle angle for the end range of the damper, or dorsiflexion stop). In addition, the potential loss of energy from the damping of both the Echelon and Motionfoot may limit the ability of these prostheses to store and return energy to the user.
The Proprio Foot (Össur, Foothill Ranch, Calif., USA) actively adapts to surface orientation. It senses changes in surface orientation then actuates appropriate changes to effective prosthesis alignment. One of the limitations of the Proprio Foot is the timing of the adaptation. Adaptation occurs following the step where the sloped surface was detected, during swing phase. In addition, the change is incremental, thus a significant change in slope could require several steps before full adaptation is achieved. Another limitation of the Proprio Foot is its high cost.
In the real world, surfaces can change slope rapidly. Able-bodied persons are able to adjust their limb properties prior to encountering a new surface (see Ferris, D., Liang, K., and Farley, C., 1999, “Runners Adjust Leg Stiffness for Their First Step on a New Running Surface,” J. Biomech., 32(8), pp. 787-794; Prentice, S., Hasler, E., Groves, J., and Frank, J., 2004, “Locomotor Adaptations for Changes in the Slope of the Walking Surface,” Gait & Pos., 20(3), pp. 255-265). Yet uneven surfaces with rapidly changing slope could result in a Proprio Foot adapted for a decline when the user steps onto an incline or vice versa. Recent studies involving the Proprio Foot on sloped surfaces have had to preset the device to a “fully adapted” state based on the onboard adaptation algorithms and the surface slopes used in the studies to avoid this problem (Alimusaj, M., Fradet, L., Braatz, F., Gemer, H., and Wolf, S., 2009, “Kinematics and Kinetics With an Adaptive Ankle Foot System During Stair Ambulation of Transtibial Amputees,” Gait & Pos., 30(3), pp. 356-363; Fradet, L., Alimusaj, M., Braatz, F., and Wolf, S., 2010, “Biomechanical Analysis of Ramp Ambulation of Transtibial Amputees with an Adaptive Ankle Foot System,” Gait & Pos., 32(2), pp. 191-198; and Wolf, S., Alimusaj, M., Fradet, L., Siegel, J., and Braatz, F., 2009, “Pressure Characteristics at the Stump/Socket Interface in Transtibial Amputees Using an Adaptive Prosthetic Foot,” Clin. Biomech., 24(10), 860-865).
Despite significant advances in prosthetic technology in recent years, commercially available lower limb prosthetic devices are as yet unable to provide biomimetic surface slope adaptation at the ankle on every step. Such examples may also include the BIOM (iWalk, Bedford, Mass., USA).
The prior art demonstrates that there is a current and long-felt need for an improved ankle prosthesis or ankle-foot prosthesis or orthosis that can better emulate the gait of an able-bodied individual and adapt to the terrain on the first step.